OSI Model

OSI Model (Open Systems Interconnection Model) is simply a reference tool, a standardised Reference Framework for conceptualising data communications between networks.OSI Model

OSI was the first conceptual model that characterized and standardized the communication functions of a Network system without regard to its underlying internal structure and technology.

OSI is a Vendor Neutral Model. Before OSI Model, each vendor used to use its own network model to manufacture Network equipment. So, one vendor’s equipment & network was not compatible with another vendor’s equipment. Buyers were bound to buy only from one vendor.

For example, if you were buying IBM Computers then you were now bound to buy all Network Equipment from IBM as well in the future. Because IBM Computers couldn’t be connected by other vendor Switches or Routers. You must buy IBM Switches to connect them. They will not work with HP Server or Cisco Switches.

OSI Model

To resolve this problem, the idea of a standardised Network Model emerged. International Organization for Standardization (ISO) started working on it, different network models were proposed and finally the OSI Model was chosen & released in 1984 by ISO. OSI provided brought in the flexibility along with standardization.

In simple words, OSI reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer.

*An important point worth of mentioning here is that: Although OSI is the standard today but we never find it on real equipment. The model in action today is TCP/IP. OSI is used only for standardization & reference purposes.

 

Layers in OSI Model

OSI Model divides the network communication processes into seven layers in order to simplify it. Each layer performs specific functions to support the layers above it … which we will discuss in detail later. OSI Model has 7x Layers starting from Physical till Application level as in below:OSI Model

Physical Layer standardises the Physical medium. For example, signal level, voltage, radio, cabling standards & so on…. It simply, standardises everything which you can touch & feel. Physical layer is responsible for ultimate transmission of digital data bits in the form of electrical, optical or radio signals from the Physical layer of the source or sending device over network communications media to the Physical layer of the receiving of destination device.  Data Link Layer standardizes the Physical Addressing. It is responsible for Data Switching. Physical addressing includes MAC Address & Ethernet Frames. Network Layer deals with Logical Addressing. It is responsible for Data Routing. Transport Layer ensures the error free data transmission across networks. It mainly focuses on data delivery. Transport Layer mainly uses two protocols called TCP & UDP which we will discuss in detail in later sections.  Session Layer is responsible for session control. In simple words, it manages on how to start, control, and end conversations between two hosts or computers.  Presentation Layer is responsible for data organization & conversion so that it is readable by Application layer of all hosts/applications involved in the data communication.  Application Layer is responsible for end-user processes or applications. It supplies network services to end-user applications.

Written by 

Waqas Karim is a seasoned Network Expert … Geek. He is the founder of ATech. ATech was started for learning & sharing. Over time the platform has grown to include other resources which continue to attract fellow networkers. Today it sees upward trend of a hundred thousand visitors per month, scattered all over the globe. His specialty is networking, but his interest & expertise spans from traditional IT to Network Security including Programming, Virtualization, Service Provider & so on... no matter the badge on the box. He is CCIE Certified (CCIE#56732) in addition to below badges: Telecom Engineer (BE) + CCIE-RS (CCIE#56732) + Huawei Certified (HW#706632) + MBA Microsoft Certified MCITP, MCSE#109*26, CCNP-Sec, CCNP-SP, CCNA-DC, CompTIA Security+, Nokia NRS-I, JNCIA, ITIL Certified (ITIL#*6373), CEH (Certified Ethical Hacker).

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Isabelle.Lily@yahoo.com
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Isabelle.Lily@yahoo.com

How is the multi-tasking done in network equipment & on which layer to be precise?

Dylan
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Detailed explanation … like it!

Noahn
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How many type of port numbers are there on transport layer in total?

Adrian_allan@googlemail.com
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Adrian_allan@googlemail.com

What does Transport Layer do exactly?

Ava Ellan
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What is CRC? On which layer does CRC work?

Zoe.A
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So, the real model we use is TCP/IP model?

LilyHarper7798@ymail.com
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LilyHarper7798@ymail.com

What is the different b/w TCP & UPD header?

Sophie
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Really nice & concise post. Keep it up )):

Fabian
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Great graphics!!!

Luis Diego
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What are media layers or upper layers? Are these OSI layers or TCP/IP layer names?